The invention concern a system of rolls in a supercalender, comprising a variable-crown upper roll adjustable in zones, a variable-crown lower roll adjustable in zones, several intermediate rolls placed between the upper roll and the lower roll, as well as a hydraulic control system for the system of rolls.
During operation, the system of rolls in a supercalender is deflected, which results mainly from the journalling of the intermediate rolls in the system of rolls. This is why, at present, in supercalenders, variable-crown rolls, in particular rolls adjustable in zones, are used commonly as the upper and lower rolls. In principle, the variable-crown rolls and the rolls adjustable in zones may be either stationary, i.e. rolls in which the roll mantle is mounted at its ends relative the roll shaft by means of bearings so that the positions of the ends of the mantle cannot be changed relative the shaft, but in the area between the ends, the roll mantle can be deflected relative the roll shaft, or then, the rolls may be floating, i.e. rolls in which the roll mantle can also move at its ends relative the roll shaft in the direction of compression.
In a supercalender, a solution is known in prior art wherein a stationary variable-crown roll is used as the upper roll and as the lower roll in the system of rolls. In such a system of rolls, however, a considerable drawback consists of vibrations of the system of rolls, which are seen as inferior paper quality. The properties of attenuation of vibrations of a system of rolls are quite poor if both the upper roll and the lower roll are stationary variable-crown rolls.
Moreover, in prior art, solutions are known in which both the upper roll and the lower roll are floating variable-crown rolls, or in which the upper roll is floating and the lower roll is stationary. A floating upper roll, however, involves the considerable drawback that, when the roll shaft is loaded by means of external loading means, the upper roll cannot be made straight, because at the ends of the upper roll there are no bearing loads. In such a case, the distribution of the linear load at the upper roll cannot be made straight, and the profile of the linear load cannot even be made sufficiently accurate. Moreover, some paper qualities require relatively low levels of linear load, in which case the mantle of a floating upper roll cannot be placed in the correct position within the zones, but often the nip between the upper roll and the topmost intermediate roll may even remain open in the lateral areas. In connection with a floating upper roll, attempts have been made to correct the deflection by means of counter-zones provided in the area of the ends of the roll mantle, the direction of effect of said counter-zones being opposite to the direction of compression. However, the deflection cannot be corrected by means of the counter-zones either, because the roll mantle is very stiff.
A floating upper roll further involves the drawback that the upper roll must be positioned from time to time, e.g., owing to leakages in the hydraulic pressure cylinders acting as external loading means. Moreover, positioning must, of course, be carried out always when the size of the intermediate rolls is to be changed. Moreover, the requirements of accuracy of the positioning are very high, and since the positioning is carried out by means of hydraulic cylinders, these form a critical point in view of the hydraulic system of the set of rolls, because the pressures effective in the hydraulic cylinders are very high.